A plurality of computers can be connected together to constitute LAN (Local Area Network). In LAN, information in the form of file, data, and the like as well as peripheral devices, such as a printer, can be shared. Further, information exchange, including transfer of electronic mail and data contents, can be carried out.
Conventionally, LANs (Local Area Networks) have been usually set up as wired networks using optical fibers, coaxial cables, or twisted pair wires. In this case, line installation work is required and this makes it difficult to readily set up a network and complicates cable routing. Even if LAN is set up, the moving ranges of equipment are limited by cable length and this makes the LAN inconvenient. For this reason, as a system which liberates users from the troublesomeness of wiring of conventional wired LANs, wireless LAN has received attention. With wireless LAN, most wires and cables in work spaces, such as offices, can be omitted and such communication terminals as personal computers (PCs) can be relocated with comparative ease.
Recently, demands for wireless LAN systems have sharply grown as their speed is enhanced and their prices are reduced. More recently, introduction of personal area networks (PANs) has been considered. This is a small-scale wireless network set up for information communication among a plurality of pieces of electronic equipment present by people's side. For example, varied radio communication systems are defined which use frequency bands (e.g. 2.4-GHz band and 5-GHz band) requiring no permission from competent authorities.
For example, IEEE802.15.3 is performing standardizing activities for fast wireless personal area networks over 20 Mbps. The responsible section is forging ahead with standardization of a network based on the PHY layer using signals mainly in the 2.4-GHz band.
In this type of wireless personal network, one radio communication device operates as a control station called “coordinator.” A personal area network is setup with in a range of 10 or less meters from the coordinator at the center. The coordinator transmits beacon signals with a specified cycle and this cycle of beacons is taken as a transmission frame cycle. In each transmission frame cycle, time slots used by individual radio communication devices are allocated.
One of allocating methods for time slots which are presently adopted is, “Guaranteed Time Slot (GTS).” This assumes such a communication method that a required capacity of transmission is guaranteed and yet transmission bands are dynamically allocated.
For example, for the MAC layer standardized by IEEE802.15.3, contention access period (CAP) and contention free period (CFP) are provided. When asynchronous communication is conducted, contention access periods are used to exchange short data or command information. When stream communication is conducted, time slots are dynamically allocated by guaranteed time slot (GTS) within contention free periods and bandwidth reservation transmission is made.
The MAC layer portion standardized by IEEE802.15.3 is so defined that the portion is applicable not only to the PHY layer using signals in the 2.4-GHz band but also as a standard specification for other PHY layers. Further, for the PHY layer standardized by IEEE802.15.3, standardizing activities are being started to use other PHY layers than the PHY layer using signals in the 2.4-GHz band.
Recently, wireless LAN (Local Area Network) systems using SS (Spread Spectrum) have been put into practical use. Further, the UWB (Ultra Wide Band) transmission method to which SS is applied has been proposed for applications for PAN and the like.
DS (Direct Spread) is a type of SS. In DS, at the transmitting end, information signals are multiplied by a series of random code called PN (Pseudo Noise) code to spread the occupied band. At the receiving end, the received spread information signals are multiplied by the PN code to de-spread and the information signals are thereby reproduced. The UWB transmission method is such that the rate of spreading of information signals is increased to the maximum. In the UWB method, data is spread, for example, from 2 GHz to as ultra high a frequency band as 6 GHz in transmission/reception, and high-speed data transmission is thereby accomplished.
In UWB, trains of impulse signals having as very short a period as several hundred picoseconds are used to constitute information signals, and trains of these signals are transmitted and received. Its occupied bandwidth is on the order of gigahertz and the value obtained by dividing the occupied bandwidth by its center frequency (e.g. 1 GHz to 10 GHz) is substantially equal to 1. This bandwidth is ultra wide even as compared with bandwidths usually used in wireless LAN based on so-called W-CDMA, cdma2000, SS (Spread Spectrum), or OFDM (orthogonal Frequency Division Multiplexing).
FIG. 17 illustrates an example of data transmission using UWB. Inputted information 901 is spread by a spreading sequence 902. In some systems using UWB, this multiplication of spreading sequence may be skipped.
The information signal 903 which underwent spread spectrum is modulated using impulse signals in UWB (wavelet pulses) (905). Modulation methods under study include PPM (Pulse Position Modulation), phase modulation (biphase modulation), and amplitude modulation.
The impulse signals used in UWB are very thin pulses; therefore, they use very wide bands in terms of frequency spectrum. As a result, inputted information signals have only power at the noise level or lower levels in each frequency domain.
In the figure, the Rx signal 905 is fraught with noises; however, it can be detected by computing the value of correlation between the Rx signal and the impulse signal. Further, in many systems, signal spreading is performed, and many impulse signals are transmitted for one bit of transmitted information. Therefore, the reception correlative value 907 of the impulse signal can be integrated by an amount equivalent to the spreading sequence length (908). As a result, detection of the Tx signal is further facilitated.
Signals spread by the UWB transmission method have only power at the noise level or lower levels in each frequency domain. On this account, communication systems using the UWB transmission method are comparatively easy to make to coexist with communication systems based on the other methods than UWB.
According to the specifications for the PHY layer using signals in the 2.4-GHz band, standardized by IEEE802.15.3, a plurality of radio communication systems exist in the same frequency band. Therefore, compatibility with these systems must be taken into account.
Meanwhile, trains of impulse signals used in the UWB radio communication method do not have a specific frequency carrier and carrier sense is difficult to perform thereon. Therefore, if the UWB radio communication method is applied as the PHY layer of IEEE802.15.3, a problem arises. Since there is not a specific carrier signal, it is comparatively difficult to exercise access control using carrier sense standardized by the section (Carrier Sense Multiple Access). In such cases, access control by time-division multiplexing is often resorted to.
This involves a problem that it is difficult to provide contention access periods (CAP) in the MAC layer standardized by IEEE802.15.3. At a radio communication device as the source of data transmission, a procedure must be followed even if asynchronous communication is conducted. The procedure is such that bandwidth reservation is made in a contention free period (CFA) before information transmission is made. This causes significant delay in transmission processing.
In conventional access control methods using carrier sense, a radio communication device as the destination of data transmission must always wait for reception in asynchronous communication. This brings a great disadvantage in terms of power consumption, especially, where a communication device is constituted as battery-operated equipment such as portable terminal. These problems do not arise only where hierarchic topology (e.g. “control station” and “communicating station” controlled by the control station) is constructed like the MAC layer defined by IEEE802.15.3. The problems of the same kind also arise where flat topology without a control station controlling a network, like ad hoc network, is constructed.